This invention relates generally to antenna horns and, more particularly, to antenna horns having multiple sections. Horn antennas are widely used in microwave communication systems. Basically a horn antenna is a flared structure that provides coupling between free space and a waveguide used to carry microwave signals, either received from the antenna or to be transmitted to the antenna. Although some microwave energy could be radiated from or received in an open-ended waveguide, flaring the open end of the waveguide results in better impedance matching between the waveguide and free-space. This flared horn antenna structure provides more efficient coupling both in transmit and receive modes. The efficiency of antenna horns is particularly important in the design of phased arrays of multiple antenna elements, each with its own antenna horn.
The gain of an antenna array is dependent on two factors: the element gain (which depends on the element aperture efficiency) and the number of elements in the array. To satisfy an overall array gain requirement it is desirable that the element aperture efficiency be as high as possible. Using a high efficiency antenna element would allow the number of radiating elements needed in the array to be reduced, thus reducing the array's overall size and weight. More importantly, for an active array the number of active circuit modules, such as solid-state power amplifiers, phase-shifters and band-pass filters, is also reduced by the use of high efficiency antenna elements. These circuit modules constitute the most expensive parts of a phased array system, and minimizing the number of modules results in a significant cost savings. For instance, a 10% aperture efficiency improvement in each radiating element allows the use of 10% fewer elements, which in turns reduces array component costs by 10%. Accordingly, there always exists a need for an antenna horn with improved aperture efficiency. The antenna element gain can be increased by simply increasing the aperture size, because the antenna gain is proportional to the area of the aperture. This is not, however, a practical approach in the design of antenna array. Increasing the aperture area results in increased weight and cost of the array.
It is well known that the presence of a particular set of TE (transverse electric) modes with proper amplitude and phase yields a uniform aperture distribution, resulting in a high aperture efficiency. Prior to the present invention, attempts to improve aperture efficiency by the use of a stepped horn profile have suffered from high fabrication costs and limited bandwidth. Thus, there is still a significant need for a new approach to antenna horn design that increases aperture efficiency over a wider bandwidth to allow for fewer antenna elements in an array. The present invention satisfies this need.